This invention relates to an improved method for manufacturing individual slide fastener elements from a metal strip by press working.
Heretofore, various methods for manufacturing individual slide fastener elements by press working have been suggested, the slide fastener elements made by each of these methods include interlocking projections and a pocket on each side of the top and bottom surfaces of a head portion thereof and a pair of spaced leg portions adapted to be clamped on the beaded edge of the fastener tape. In a typical method of such methods, the interlocking projections are directly formed by embossing portions of the flat top and bottom surfaces of a metal strip. By this embossing method, however, it is impossible to provide projections of sufficient height even though the thickness of the metal strip is increased and the fastener chain thus obtained does not have sufficient interlocking force. Also, in this method, if the thickness of the strip is increased, the pitch for attaching the fastener elements on the fastener tape is correspondingly increased, because the thickness of the leg portions of the fastener element corresponds to the increase of the strip and this increases the resistance to the sliding movement of the slider over the fastener chain, the flexibility of the fastener chain is greatly diminished and further more material is required for producing the fastener elements. On the other hand, if the thickness of the strip is decreased, the height of the embossed projections is correspondingly decreased, and thus, the interlocking force of the fastener chain is decreased. In another method, the metal strip from which the fastener elements are formed includes a medial ridge on each side of the top and bottom surfaces thereof and the interlocking projections and pockets are formed by deforming such ridges. One example of such method is disclosed in U.S. Pat. No. 3,136,046. In this method, however, as the interlocking projections are regularly spaced on the strip, the portions of the ridges which do not contribute to the formation of the projections must be depressed to the level of the strip. When the portions of the ridges are depressed, the web portion of the strip is concurrently pressed and the width of the strip is correspondingly increased. To prevent this, the ridges must be limited to a relatively low height, and, therefore, the projections are limited to a relatively low height. In another method, the fastener elements are formed step-by-step by press working from a round metal wire. One example of such method is disclosed in U.S. Pat. No. 2,622,295. However, this method includes many working steps and is very disadvantageous from a manufacturing cost standpoint. Also, in this method, the metal strip in which a series of the fastener elements are formed must be separated into the individual fastener elements by once removing the strip from the press machine and then advancing it in reverse direction in the separating machine. Thus the operation is very inefficient.